Heat exchangers having fluid conduits of relatively flat cross-section are known in the art. Such heat exchangers are often referred to as "parallel flow" heat exchangers. In such parallel flow heat exchangers, the interior of each tube is divided into a plurality of parallel flow paths of relatively small hydraulic diameter (e.g., 0.070 inch or less), to accommodate the flow of heat transfer fluid (e.g., a vapor compression refrigerant) therethrough. Parallel flow heat exchangers may be of the "tube and fin" type in which the flat tubes are laced through a plurality of heat transfer enhancing fins or of the "serpentine fin" type in which serpentine fins are coupled between the flat tubes. Heretofore, parallel flow heat exchangers typically have been used as condensers in applications where space is at a premium, such as in automobile air conditioning systems.
To enhance heat transfer between fluid such as a vapor compression refrigerant flowing inside the heat exchanger conduits and an external fluid such as air flowing through the heat exchanger, it is usually advantageous to have flow channels of relatively small hydraulic diameter. However, such small hydraulic diameters usually result in unwanted pressure drops as the fluid flows through the conduits. There is therefore a need for an improved heat exchanger to provide the advantages of relatively small hydraulic diameter flow paths, without the pressure drops which are usually associated with such relatively small hydraulic diameter flow paths.